This invention relates to improvements in forced-cooled superconductor.
Of late, there is the tendency that superconducting coils operated at temperatures of liquid helium are increased in size and operated at large currents and thus the superconducting coils are required to have high mechanical strength and electric insulation capability. There are various cooling methods for the superconducting coils, that is, for example, the pool-cooling method in which the superconductor is cooled in a liquid helium bath and the forced-cooled method in which supercritical helium is supplied to and circulated through the cooling channel in a superconductor. Although the forced-cooled method is superior to the pool-cooling method with respect to mechanical strength and electric insulation capability, the former system has the drawback that the system gives lower stability margin of the coils.
FIG. 4 shows one example of the ordinary forced-cooled superconductors, which is composed of a conduit 1 formed of metal or plastic and twisted superconducting strands 2 disposed within the conduit 1 and a coolant 3 such as helium gas, for example, is passed through the spaces between the superconducting strands within the conduit. In order to increase the stability margin of the forced-cooled superconducting coils with the ordinary geometry shown in FIG. 4, the twisted-stranded cables are disposed within the conduit to thereby increase the cooling perimeter of the stranded cables with respect to the helium gas as the coolant. However, the ordinary superconductor has the disadvantage that since the many stranded cables are disposed within the conduit, high pressure drop of helium gas as a coolant through the conductor is substantial, resulting in temperature rise and in lower coolant speed whereby a thermal disturbance externally induced in an upstream zone of the flow passage causes a so-called transition normal state in the portion of the conductor disposed in a downstream zone in the flow passage.